With a self-signed cert they would have to do a Man In The Middle attack on you to see your traffic.
They don't have the capacity (or ability! man of their fibertaps are passive) to do that to all the traffic at all the time.
The problem with making a CA-blessed cert a requirement for all or even most of the traffic, is that it forces the NSA to break SSL/TLS or CAs definitively, otherwise they cannot do their job.
Fundamentally this is a political problem, just slapping encryption on traffic will not solve it.
But it can shift the economy of the situation -- but you should think carefully what way you shift it.